1. Field of the Invention
This invention relates to novel homogeneous regenerative catalyzed packing material suitable for the regenerative catalytic oxidation of waste gases such as, but not limited to, volatile organic compounds, carbon monoxide and combinations thereof. A particular embodiment of the invention is a process for making such catalytic packing material by impregnating porous regenerative heat transfer packing material with a solution of a catalyst precursor, and then fixing the precursor into catalyst form.
2. Description of Related Art
Air pollutants, such as volatile organic compounds (VOC), carbon monoxide (CO) and oxides of nitrogen (NOx), are often controlled industrially by an incineration system that uses either a thermal or a catalytic process. Control of VOC and CO emissions is achieved by initiating oxidation reactions in these systems that convert the pollutants to harmless water and CO.sub.2. Control of NOx is often achieved by a selective reduction reaction which reacts ammonia with NOx to form N.sub.2 and water.
The abatement system is typically installed downstream of an industrial process to remove the pollutant constituents in the flue gas before the gas is emitted to the atmosphere. Thermal processes rely on homogeneous gas phase reactions for the destruction of these compounds, and normally operate at about 1500 to 1800.degree. F. (800-1000.degree. C.) with a residence time of about 1 second. On the other hand, the destruction reactions for catalytic processes occur at the catalyst surface rather than in the gas phase. Catalytic processes typically operate at about 600 to 1000.degree. F. (300-550.degree. C.) with a residence of time of about 0.1 second or less. Catalytic incineration systems are normally smaller in size, and consume less fuel than non-catalytic thermal systems.
Commercially, there are two general types of incineration designs, regenerative and recuperative, for either thermal or catalytic processes. Regenerative thermal oxidation (RTO) or regenerative catalytic oxidation (RCO) systems have very high thermal efficiency (&gt;90%). Recuperative thermal or catalytic oxidizers typically have a heat recovery of no greater than 70%. Selection of regenerative or recuperative type of oxidizers depend primarily on the exhaust concentrations and the exhaust flows, which also affect the operating and capital costs of the abatement system. A detailed discussion of VOC control methods, including regenerative and recuperative thermal oxidation and catalytic oxidation, is set forth in Ruddy, et al., "Select the Best VOC Control Strategy", Chemical Engineering Progress, July 1993, pp. 28-35, incorporated herein by reference.
A typical regenerative thermal oxidation system is described in Houston, U.S. Pat. No. 3,870,474, incorporated herein by reference. In such a process, the VOCs and CO in a gas stream are incinerated at a relatively high temperature of about 1500.degree. F. (800.degree. C.). Before entering the combustion zone, the gas stream passes through a first packed column of heat transfer packing material which heats the gas, and then exits through an identical second packed column which is heated by the gas from the combustion zone. Thus the hot gas exiting the combustion zone passes through a packed column, heating the packing material therein. Then the flow of the gas is reversed, and the incoming gas is heated as it passes through the packed column. By the use of such regenerative processes, the efficiency of thermal incineration has been greatly increased.
A drawback of such thermal oxidation systems is that they require heating the gas stream to the relatively high temperature of about 1500.degree. F. The 3,870,474 patent does indicate, at column 6,lines 3-7, that a suitable combustion catalyst may be placed in the warmest part of the regenerators to cause the contaminants in the air to be oxidized at a lower temperature.
Heat transfer packing materials are conventionally made up of inorganic metals or metal oxides. See Perry's Chemical Engineers' Handbook, Fifth Edition, 1973, Chapter 18, on Gas-Liquid Contacting. FIG. 18-35 illustrates typical packings such as Raschig rings, Lessing rings, Berl saddles, Intalox saddles, Tellerette and Pall rings. Ceramic packings can be almost any shape, including balls, rings or saddles. Packings are also available in a number of different sizes. Smaller-sized packings have a higher heat transfer efficiency due the higher geometric surface are per unit reactor volume, but a higher pressure drop as well. The optimal packing size and reactor dimensions are chosen to match the requirements of the auxiliary system components, such as blowers, fans, duct dimensions, etc.
A great advantage of using catalysts made of heat transfer packing materials is that the catalyst bed itself is also the effective source of heat storage for the regenerative heat transfer. Thus, regenerative systems incorporating these catalyst materials will inherently have reduced total bed dimensions than those systems that use catalysts of poor heat transfer/storage materials. Further, the shapes of heat transfer packing materials are all optimized to provide low pressure drop, and high heat transfer efficiency. These same properties are key catalytic surface characteristic to achieve high mass transfer efficiency for catalytically oxidizing VOC emissions with minimal pressure loss. These unique features make the use of heat transfer packing materials as catalyst carriers highly desirable for regenerative catalytic oxidizers.
In a regenerative bed, the heat transfer packing materials are typically laid down randomly into the vessel. The packing materials normally are required to have sufficient physical strength to retain bed weight for the particular packing and vessel involved. This physical strength is generally indicated by the crush strength of the packing, which can be measured by putting a sample of the packing in a standard compression testing device, and measuring the force needed to break the packing in its weakest orientation. Additionally, these packing materials need very high cohesive strength to resist erosion that may be caused by interparticle abrasion, loading and unloading, etc, and adhesive strength to retain catalyst bound to the surface. For catalyzed heat transfer packings, the erosion resistance is particularly important as erosion is a key likely cause for the deactivation of the catalyst effectiveness.
European Patent Application No. 629432, published Dec. 12, 1994, describes a heat transfer packing material with catalyst and/or adsorbent on its surface for use in a regenerative incineration process. In this publication, the catalyst is applied as a slurry washcoat to the exterior of low porosity heat transfer packing, such as ceramic saddles. As described in the publication, the catalyst ingredients in such washcoats are supported on high surface area inorganic oxide powders which are in turn deposited on the surface of the ceramic substrates. However, the slurry washcoats have been found to adhere poorly to the heat transfer packing materials, typically ceramic substrates. Under normal operating conditions, these slurry coated catalyst materials are prone to deactivation due to attrition.
Another known method of placing catalyst onto any support is by solution impregnation, in which the catalysts is impregnated from a solution into the pores of the support material. However, most existing heat transfer packing materials do not have required surface properties to allow such impregnation and to provide high catalytic activity. The key reason is that commercial packing materials are normally very dense, and lack the microstructure needed for catalytic activity and the porosity needed to allow solution impregnation. This is due, in part, to the need of high physical strength of such packing materials. To obtain such strength, packing materials are typically precalcined at elevated temperatures which results in the loss of porosity and the collapse of micro surface area structure. As a result, a catalyst of high physical strength combined with high catalytic activity has not been commercially available.